1. Field of the Invention
The present invention relates to a multimedia broadcast/multicast service (MBMS) of a UMTS (Universal Mobile Telecommunications System) and, more particularly, to a method for scheduling transmission of MBMS data.
2. Description of the Background Art
A universal mobile telecommunications system (UMTS) is a third generation mobile communication system that has evolved from a standard known as Global System for Mobile communications (GSM). This standard is a European standard which aims to provide an improved mobile communication service based on a GSM core network and wideband code division multiple access (W-CDMA) technology.
FIG. 1 shows a network structure of a general UMTS.
As shown in FIG. 1, the UMTS is roughly divided into a terminal, a UTRAN and a core network.
The UTRAN includes one or more radio network sub-systems (RNS). Each RNS includes an RNC and one or more Node Bs managed by the RNCs.
Node Bs managed by the RNCs, receive information sent by the physical layer of a terminal (e.g., mobile station, user equipment and/or subscriber unit) through an uplink, and transmit data to a terminal through a downlink. Node Bs, thus, operate as access points of the UTRAN for terminal.
The RNCs perform functions which include assigning and managing radio resources, and operate as an access point with respect to the core network.
A primary function of UTRAN is constructing and maintaining a radio access bearer (RAB) for a call connection between the terminal and the core network. The core network applies quality of service (QoS) requirements of end-to-end to the RAB, and accordingly, UTRAN can satisfy the QoS requirements of the end-to-end by constructing and maintaining the RAB.
The RAB service is divided into an lu bearer service and a radio bearer service of a lower concept. The lu bearer service handles reliable user data transmission between boundary nodes of UTRAN and the core network, while the radio bearer service handles reliable user data transmission between the terminal and UTRAN.
In the UMTS, traffic is classified as four QoS classes according to its characteristics as follows: conversational class, streaming class, interactive class and background class. The conversational class is a type of symmetric traffic requiring minimum fixed delay, allowing no buffering and offering guaranteed bit rate. The streaming class is a type of an asymmetric traffic requiring minimum variable delay, allowing buffering and offering guaranteed bit rate. The interactive class is a type of an asymmetric traffic requiring moderate variable delay, allowing buffering and offering no guaranteed bit rate. The background class is a type of an asymmetric traffic requiring big variable delay, allowing buffering and offering no guaranteed bit rate
FIG. 2 illustrates a radio protocol between the terminal and UTRAN on the basis of the 3GPP wireless access network standards.
With reference to FIG. 2, the radio protocol is vertically formed of a physical layer, a data link layer and a network layer, and is horizontally divided into a user plane for transmitting data information and a control plane for transmitting a control signal.
The user plane is a region to which traffic information of a user such as voice or an IP packet is transmitted. The control plane is a region to which control information such as an interface of a network or maintenance and management of a call is transmitted.
In FIG. 2, protocol layers can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model well known in a communication system.
The first layer (PHY) provides an information transfer service to the upper layer by using various radio transfer techniques.
The first layer is connected to the MAC layer through a transport channel, and data is transferred between the MAC layer and the PHY layer through the transport channel.
The MAC layer provides a re-allocation service of the MAC parameter for allocation and re-allocation of radio resources.
The MAC layer is connected to the radio link control (RLC) layer through a logical channel, and various logical channels are provided according to the kind of transmitted information. In general, when information of the control plane is transmitted, a control channel is used. When information of the user plane is transmitted, a traffic channel is used.
The MAC is classified into an MAC-b sublayer, an MAC-d sublayer and an MAC-c/sh sublayer according to types of managed transport channels. The MAC-b sublayer manages a BCH (Broadcast Channel) handling broadcast of system information, while the MAC-c/sh sublayer manages common transport channel such as FACH (Forward Access Channel), DSCH (Downlink Shared Channel), or the like, common to other terminals.
In UTRAN, the MAC-c/sh sublayer is positioned at a controlling RNC (CRNC) and manages channels shared by every terminal in a cell, so that one MAC-c/sh sublayer exists in each cell.
The MAC-d sublayer manages a DCH (Dedicated Channel), a dedicated transport channel for a specific terminal. Accordingly, the MAC-d sublayer is positioned at a serving RNC (SRNC) managing a corresponding terminal, and one MAC-d sublayer exists also at each terminal.
A radio link control (RLC) layer supports a reliable data transmission and may perform a function of segmentation and concatenation of an RLC service data unit (SDU) coming from a upper layer. The RLC SDU transferred from the upper layer is adjusted in its size according to a throughput capacity at the RLC layer, to which header information is added, and then transferred in a form of a PDU (Protocol Data Unit) to the MAC layer. The RLC layer includes an RLC buffer for storing the RLC SDU or the RLC PDU coming from the upper layer.
A broadcast/multicast control (BMC) layer performs functions of scheduling a cell broadcast message (CB) transferred from the core network and broadcasting the CB to UEs positioned in a specific cell(s). At the side of UTRAN, the CB message transferred from the upper layer is combined with information, such as a message ID, a serial number or a coding scheme, and transferred in a form of BMC message to the RLC layer and to the MAC layer through a CTCH (Common Traffic Channel), a logical channel. In this case, the logical channel CTCH is mapped to a FACH (Forward Access Channel), a transport channel, and an S-CCPCH (Secondary Common Control Physical Channel), a physical channel.
A packet data convergence protocol (PDCP) layer is an upper layer of the RLC layer, allowing data to be transmitted effectively on a radio interface with a relatively small bandwidth through a network protocol such as the IPv4 or the IPv6. For this purpose, the PDCP layer performs a function of reducing unnecessary control information, which is called a header compression, and in this respect, RFC2507 and RFC3095 (robust header compression: ROHC), a header compression technique defined by an Internet standardization group called an IETF (Internet Engineering Task Force), can be used. In these methods, because the only information requisite for the header part of a data is transmitted, control information is transmitted, so that an amount of data transmission can be reduced.
The RRC layer positioned in the lowest portion of the third layer (L3) is defined only in the control plane and controls the logical channels, the transport channels, and the physical channels in relation to the setup, the reconfiguration, and the release of the RBs. The RB signifies a service provided by the second layer for data transmission between the terminal and UTRAN, and setting up the RB means processes of stipulating the characteristics of a protocol layer and a channel, which are required for providing a specific service, and setting the respective detailed parameters and operation methods.
A cell broadcast service (CBS) related to the BMC layer will now be described.
A service that a message including a character or a numeric is given and taken between terminals or between a terminal and a network is called a short message service (SMS). The SMS is classified into a cell broadcast SMS (SMS-CB) for transmitting the same message to one or more cells and a point-to-point SMS (SMS-PP).
The CBS service, which broadcasts plural CBS messages to every user of a specific area, corresponds to the SMS-CB.
CBS messages are broadcast to a geographical area called a broadcast area. This area includes one or more cells or is wholly constructed as a public land mobile network (PLMN). Each CBS message is broadcast to the geographical area according to a mutual contract between an information provider and a PLMN operator.
The BMC messages used at the BMC protocol includes a CBS message for transmitting user information, a scheduling message for allowing a terminal to easily receive a CBC message, and a CBS41 message for transmitting a short message service (SMS) message transmitted from an ANSI41 network. Every message is transmitted from UTRAN only to the terminal. The terminal performs a discontinuous reception (DRX) function by using information of the scheduling message transferred from UTRAN, thereby reducing a battery consumption.
The scheduling of a BMC message is divided into a first level scheduling and a second level scheduling. The first level scheduling determines (allocates) a frame for transmitting a data of a common traffic channel (CTCH).
FIG. 3 illustrates a first level scheduling method. The numbers of the first line indicates a system frame number (SFN) value.
The logical channel CTCH is mapped to the physical channel (S-CCPCH) through the transport channel (FACH). The first level scheduling previously designates a frame of the physical channel by which data of the logical channel (CTCH). Data transmitted to the CTCH is transmitted by a certain consecutive M number of frames, and the frame group is repeated by certain period (N) of frames.
For example, as shown in FIG. 3, data transmitted by the CTCH is always transmitted for two consecutive frame intervals and repeated by the period of six frames. A frame group transmitting the CTCH data starts when the SFN value is ‘K’ and is repeated by the period (N). In FIG. 3, the frame group starts when the SFN value is ‘2’ and is then repeatedly transmitted by the period of ‘6’.
The first level scheduling is performed in the same manner on every service of the CBS. That is, as for every service of the CBS, the same cell is allocated with the same frame. The first level scheduling is performed at the RRC layer and the values N, K and M are included in the system information and broadcast to the terminal.
The second level scheduling divides the frame allocated in the first level scheduling into a CBS schedule period. The second level scheduling is performed at the BMC layer, and the terminal obtains CBS schedule period information by receiving a BMC scheduling message. The BMC scheduling message includes information on a length of the CBS schedule period and a start point of the CBS schedule period.
The length of the CBS schedule indicates a length between a start and an end of the CBS schedule period starting after the BMC scheduling message. The start point of the CBS schedule period indicates a different value between a transmission time point of a current BMC scheduling message and a start point of a CBS schedule period starting after the BMC scheduling message.
Thus, as the terminal receiving the CBS message receives the BMC scheduling message, it can recognize when the next CBS schedule period starts and ends. The terminal can obtain the next CBS schedule period information by receiving the BMC scheduling message during the CBS schedule period. In this manner, the terminal can recognize when the BMC message is not transmitted, so that it can perform the discontinuous reception (DRX).
FIG. 4 shows a construction of the BMC scheduling message.
As shown in FIG. 4, the BMC scheduling message provides information on one or plural BMC messages transmitted during the next CBS schedule period. A new message bit map parameter indicates each message transmitted during the next CBS schedule period is a new message or a previously broadcast message. A message explanation parameter indicates a type or an ID of each BMC message transmitted during the next CBS schedule period. At this time, the message refers to one of a CBS message, a scheduling message and a CBS41 message.
The maximum length of the CBS message is generally limited to 1230 octet, so that the CBS message is not suitable for broadcasting or multicasting multimedia data. In addition, because the CBS message is broadcast to every terminal existing in a specific cell, a multicast for providing a service to a specific terminal group is not available wirelessly.
Therefore, in order to broadcast or multicast the multimedia data to a specific terminal group wirelessly, a new service called a multimedia broadcast/multicast service (MBMS) has been proposed.
The MBMS will now be described.
The MBMS is a service for transmitting multimedia data such as audio, video or image data to plural terminals by using a unidirectional point-to-multipoint bearer service. The MBMS is divided into a broadcast mode and a multicast mode. That is, the MBMS is divided into an MBMS broadcast service and an MBMS multicast service.
The MBMS broadcast mode is a service for transmitting multimedia data to every user in a broadcast area. The broadcast area means a broadcast service available area. One or more broadcast areas may exist in one PLMN, one or more broadcast services can be provided in one broadcast area, and one broadcast service can be provided to several broadcast areas.
The MBMS multicast mode is a service for transmitting multimedia data only to a specific user group existing in a multicast area. The multicast area means a multicast service available area. One or more multicast areas can exist in one PLMN, one or more multicast services can be provided in one multicast area, and one multicast service can be provided to several multicast areas.
In the multicast mode, a user is requested to join a multicast group to receive a specific multicast service. At this time, the multicast group refers to a user group that receives the specific multicast service, and joining refers to a behavior of being admitted to the multicast group intending for receiving the specific multicast service.
Two types of logical channels are provided for MBMS data transmission: MBMS Control Channel (MCCH) and MBMS Traffic Channel (MTCH). The MCCH channel is used to transmit MBMS control information to MBMS terminals. The MTCH is used to transmit data of a specific MBMS service to MBMS terminals. Therefore, if multiple MBMS services are available in a cell, multiple MTCH should be configured in the cell. In case of the multicast mode, the MTCH is configured only at the MBMS terminals that have joined the specific MBMS service.
The MAC layer adds an MAC header to the received MBMS data and transfers it to the physical layer of the base station through the common transport channel. And then, the MBMS data undergoes coding and modulation in the physical layer and is transmitted to the terminal through the common physical channel. At this time, the common transport channel transfers the data by a certain time unit of Transmission Time Interval (TTI), and the common physical channels mapped to the common transport channel are transmitted by the frame unit.
In order to simultaneously receive several services transferred through plural channels, the most simple method is to receive several channels simultaneously, which, however, makes the terminal complicate.
Thus, in order to solve the problem, in the conventional CBS service, a scheduling method is adopted in which a time during which a specific service is provided is predetermined and informed. This method is useful in case of simultaneously receiving the MBMS and a different service or the MBMS and control information. In addition, while the MBMS is not being received, the terminal may perform a discontinuous reception (DRX) to prevent battery consumption.
However, in the case of the conventional CBS service, only one service is provided in one cell so the same scheduling parameter is set in the cell. Meanwhile, in the case of the MBMS, various services are to be provided in one cell, so a different scheduling method from the scheduling of the CBS is required.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.